Motor vehicle with press-hardened visible steel component

Abstract

The invention relates to a motor vehicle in which at least one outer skin part or one semi-visible part is made of a coated press-hardened steel, the coating of which prior to heating and press-hardening contains, by weight, from 8% to 12% silicon, no more than 3% iron and no more than 0.1% unavoidable impurities, the balance being aluminum, and in which at least one outer skin part or one semi-visible part is made of a coated press-hardened steel. The coating on each side has a thickness of 10 [mu] m to 20 [mu] m.

Description

Technical Field

[0001] The present invention relates to a process for producing coated steel press-hardened parts having an improved appearance, and more particularly, but not limited to, the invention is intended for use in producing exposed or semi-exposed parts for automotive applications. Background Art

[0002] In recent years, the use of coated steels in hot stamping processes for component forming has become important, especially in the automotive industry. The manufacture of such components may include the following main steps:

[0003] -Coating of steel sheets by hot dipping

[0004] - Trimming or cutting to obtain blanks

[0005] -Heating the blank to transform the steel microstructure into austenite.

[0006] -Hot forming followed by rapid cooling of the component to obtain a predominantly martensitic structure.

[0007] Press-hardened steel parts intended for use in the manufacture of automobiles are usually coated with an aluminum-based metal coating that can itself withstand both the austenitizing heat treatment and the subsequent press-hardening step. After hot deformation and quenching of the part, the coating provides corrosion protection. The coating is deposited by hot-dip coating in a liquid bath.

[0008] Press-hardened steel parts intended for the manufacture of automobiles can be deep drawn at high temperatures and quenched in the forming tool to achieve the target microstructure. In terms of material properties, tensile strengths of 500 MPa to 2000 MPa and tensile elongations of 5% to 15% can be achieved. The main advantage of press-hardened steel parts is the combination of good formability with very high strength.

[0009] The press-hardened parts are then assembled to form a body in white which is then coated with at least one paint layer, thereby providing improved corrosion protection.

[0010] The surface appearance of press-hardened steel parts is still poor compared to the appearance obtained by cold stamping galvanized steel materials. The paint layer tends to reduce surface irregularities. But even after painting, press-hardened parts cannot be used for outer skin parts due to surface defects and the corresponding detrimental appearance. This is because press-hardened coated steel parts have various defects, such as wavy surfaces. After painting, the parts will have an unacceptable appearance, for example, locally similar to "orange peel".

[0011] With the exception of outer skin parts, semi-visible parts are only visible when the vehicle's doors are open. Press-hardened parts are not suitable for making semi-visible parts.

[0012] For the reasons mentioned above, the press-hardened parts are not directly exposed to the customer's sight but are covered by additional metal parts with better appearance. The additional parts have no or little mechanical function but have a decorative function. The additional parts act as a barrier and make the press-hardened parts invisible to the naked eye. For example, the part called body side is usually made as one large piece with a very good visual appearance. This decorative body side part acts as a barrier and covers the structural parts from the front wheel to the rear wheel. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] exist Figure 1 The common structure of a car can be seen in the figure, which is marked as follows: 1: A-pillar, 2: B-pillar, 3: C-pillar, 4: side beam, 5: roof longitudinal beam, 6: body side. Summary of the invention

[0014] The waviness W of the surface is a gentle, pseudo-periodic geometric irregularity with a longer wavelength (0.8 mm to 10 mm), which is distinguished from the roughness R corresponding to geometric irregularities with a shorter wavelength (<0.8 mm).

[0015] In the present invention, the arithmetic mean Wa of the waviness profile expressed in μm is used to characterize the surface waviness of the sheet, and the waviness measurement value with a cutoff threshold of 2.5 mm to 8.0 mm is represented by Wa. 2.5-8 To express.

[0016] The object of the present invention is therefore to provide a motor vehicle manufactured with at least one press-hardened coated steel component, the waviness Wa of which is reduced compared to the press-hardened components of the prior art. 2.5-8 Reduce.

[0017] This object is achieved by a motor vehicle according to any one of claims 1 to 7 .

[0018] To this end, the present invention discloses a process for producing a press-hardened coated steel component, the process comprising the following steps:

[0019] A) providing a steel plate, the steel plate having a thickness of 0.7 mm to 2.5 mm,

[0020] B) coating said steel sheet by hot dipping into an aluminum-based liquid metal bath containing by weight 8 to 12% silicon, not more than 3% iron and not more than 0.1% unavoidable impurities from the manufacturing process, wherein said coating has a thickness of 10 μm to 20 μm on each side,

[0021] C) temper rolling the coated steel sheet with a total elongation of 0.1% to 1.2%, the elongation being defined by the speed difference between the material entering the temper rolling stand and the material leaving the temper rolling stand,

[0022] D) cutting the coated skin-pass rolled steel sheet to obtain blanks,

[0023] E) heating the blank at a temperature of 800° C. to 970° C. to obtain a fully austenitic microstructure in the steel,

[0024] F) transferring the blank into the pressing tool,

[0025] G) press-hardening the blank to obtain a press-hardened component.

[0026] In step A), any steel can be used within the framework of the invention. However, in the case where steel with high mechanical strength is required, in particular for parts of the structure of a motor vehicle, steel with a tensile strength better than 500 MPa, advantageously between 500 and 2000 MPa, before or after heat treatment can be used. The weight composition of the steel plate is preferably as follows: 0.03% ≤ C ≤ 0.50%; 0.3% ≤ Mn ≤ 3.0%; 0.05% ≤ Si ≤ 0.8%; 0.015% ≤ Ti ≤ 0.2%; 0.005% ≤ Al ≤ 0.1%; 0% ≤ Cr ≤ 2.50%; 0% ≤ S ≤ 0.05%; 0% ≤ P ≤ 0.1%; 0% ≤ B ≤ 0.010%; 0% ≤ Ni ≤ 2.5%; 0% ≤ Mo ≤ 0.7%; 0% ≤ Nb ≤ 0.15%; 0% ≤ N ≤ 0.015%; 0% ≤ Cu ≤ 0.15%; 0% ≤ Ca ≤ 0.01%; 0% ≤ W ≤ 0.35%, the remainder being iron and unavoidable impurities from the manufacture of steel.

[0027] For example, the steel plate is 22MnB5 with the following weight composition: 0.20%≤C≤0.25%; 0.15%≤Si≤0.35%; 1.10%≤Mn≤1.40%; 0%≤Cr≤0.30%; 0.020%≤Ti≤0.060%; 0.020%≤Al≤0.060%; 0.002%≤B≤0.004%, the remainder being iron and unavoidable impurities from the manufacture of steel.

[0028] In another embodiment, the steel plate has the following weight composition: 0.24%≤C≤0.38%; 0.40%≤Mn≤3%; 0.10%≤Si≤0.70%; 0.015%≤Al≤0.070%; Cr≤2%; 0.25%≤Ni≤2%; 0.015%≤Ti≤0.10%; Nb≤0.060%; 0.0005%≤B≤0.0040%; 0.003%≤N≤0.010%; S≤0.005%; P≤0.025%; the remainder is iron and unavoidable impurities produced during the manufacture of steel.

[0029] Alternatively, the steel plate may have the following weight composition: 0.30% ≤ C ≤ 0.40%; 0.5% ≤ Mn ≤ 1.0%; 0.40% ≤ Si ≤ 0.80%; 0.1% ≤ Cr ≤ 0.4%; 0.1% ≤ Mo ≤ 0.5%; 0.01% ≤ Nb ≤ 0.1%; 0.01% ≤ Al ≤ 0.1%; 0.008% ≤ Ti ≤ 0.003%; 0.0005% ≤ B ≤ 0.003%; 0.0% ≤ P ≤ 0.02%; 0.0% ≤ Ca ≤ 0.001%; ​​0.0% ≤ S ≤ 0.004%; 0.0% ≤ N ≤ 0.005%, with the remainder being iron and unavoidable impurities resulting from the manufacture of the steel.

[0030] In another embodiment, the steel plate has the following weight composition: 0.040%≤C≤0.100%; 0.80%≤Mn≤2.00%; 0%≤Si≤0.30%; 0%≤S≤0.005%; 0%≤P≤0.030%; 0.010%≤Al≤0.070%; 0.015%≤Nb≤0.100%; 0.030%≤Ti≤0.080%; 0%≤N≤0.009%; 0%≤Cu≤0.100%; 0%≤Ni≤0.100%; 0%≤Cr≤0.100%; 0%≤Mo≤0.100%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0031] In another embodiment, the steel plate has the following weight composition: 0.06%≤C≤0.1%, 1%≤Mn≤2%, Si≤0.5%, Al≤0.1%, 0.02%≤Cr≤0.1%, 0.02%≤Nb≤0.1%, 0.0003%≤B≤0.01%, N≤0.01%, S≤0.003%, P≤0.020%, less than 0.1% of Cu, Ni and Mo, and the rest is iron and unavoidable impurities generated by the manufacture of steel.

[0032] In another embodiment, the steel plate has the following weight composition: 0.015%≤C≤0.25%; 0.5%≤Mn≤1.8%; 0.1%≤Si≤1.25%; 0.01%≤Al≤0.1%; 0.1%≤Cr≤1.0%; 0.01%≤Ti≤0.1%; 0%≤S≤0.01%; 0.001%≤B≤0.004%; 0%≤P≤0.020%; 0%≤N≤0.01%; the balance being iron and unavoidable impurities from the manufacture of steel.

[0033] Alternatively, the steel sheet has the following weight composition: 0.2%≤C≤0.34%; 0.5%≤Mn≤1.24%; 0.5%≤Si≤2.0%; 0%≤S≤0.01%; 0%≤P≤0.020%; 0%≤N≤0.01%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0034] The steel sheet can be obtained by hot rolling and optionally cold rolling, depending on the desired thickness. Thicknesses below 0.5 mm may tear during the hot forming process. Press-hardened parts with a thickness greater than 3.0 mm are not required in automobile bodies.

[0035] In step B), the steel sheet is then hot-dip coated in a molten bath and subsequently wiped with an air knife to adjust the coating thickness. If the coating thickness is less than 10 μm per side, the corrosion performance is insufficient. If the coating thickness is more than 20 μm per side, the waviness Wa of the stamped part 2.5-8 Too high.

[0036] In step C), the steel plate is subsequently subjected to skin-pass rolling. The skin-pass rolling operation takes place on a single-stand skin-pass mill, wherein the steel strip is rolled between two working rolls of said mill. Pressure is applied to the steel strip by the working rolls, which in turn apply a linear pressure along a generatrix in contact with the steel strip. The elongation at the skin-pass mill is given by the relative difference between the speed of the material rolled out of the skin-pass rolling stand and the speed of the material rolled into said stand. If the elongation is below 0.1%, noticeable surface defects will also be seen on the steel plate on the final press-hardened part. If the elongation is above 1.2%, the waviness Wa of the press-hardened part will be too high. In fact, the inventors have surprisingly found that skin-pass rolling elongations above 1.2% result in a waviness Wa of the press-hardened part. 2.5-8 Without being bound by theory, reducing the skin pass elongation also appears to reduce the waviness of the press hardened part.

[0037] Preferably, the elongation in step C) is 0.9% or less, more preferably 0.7% or less, advantageously 0.5% or less or even 0.3% or less.

[0038] In the vehicle according to the invention, the press-hardened component has a waviness Wa of less than 0.41 μm, preferably less than 0.35 μm or even less than 0.29 μm. 2.5-8 .

[0039] The press-hardened component in the automobile according to the present invention is suitable for an outer skin component.

[0040] Thanks to the invention, decorative parts that conceal the press-hardened parts can be omitted. For example, the invention allows the body side 6 to be omitted.

[0041] The press-hardened parts in the automobile according to the present invention are also applicable to semi-visible parts. For example, the present invention is applicable to the following semi-visible parts of the automobile that are only visible when the door is opened: A-pillar 1, B-pillar 2, C-pillar 3, side beam 4 or roof rail 5.

[0042] For example, the present invention is also applicable to a hatchback of an automobile including semi-visible parts that are visible only when the rear tailgate is open.

[0043] The press-hardened parts used in the automobile according to the present invention can have various types of microstructures, depending on the target mechanical properties, in particular the yield strength and the tensile strength. For example, when high resistance is required, the press-hardened parts have a steel microstructure containing at least 95% martensite by volume fraction. The press-hardened parts can also have a microstructure containing at least 50% martensite and less than 40% bainite. This is the case for parts located in the automobile that require both resistance and deformation. Allowing deformation during a collision is a design technique that absorbs the energy of the collision. Finally, for parts with an anti-intrusion function, the press-hardened parts can have a microstructure containing 5% to 20% martensite, no more than 10% bainite and at least 75% equiaxed ferrite. DETAILED DESCRIPTION

[0044] The invention will now be explained in experiments which were carried out for reference only. They are non-limiting.

[0045] Example

[0046] For all samples, the carbon steel coil used was 22MnB5. The composition of the steel was as follows: C = 0.23%; Mn = 1.2%; Si = 0.25%; %; Cr = 0.2%; Al = 0.04%; Ti = 0.04%; B = 0.003%.

[0047] All steel coils are continuously rolled to the desired thickness. After rolling, the steel coils are annealed and continuously coated with a coating deposited by hot dipping in a metal bath. The coating contains 9 wt% silicon, 3 wt% iron, the balance being aluminum.

[0048] After hot-dip aluminizing, the steel coils are skin-pass rolled at different elongation rates. The skin-pass rolling operation takes place on a single-stand skin-pass mill, where the steel strip is rolled between two working rolls of the mill. The elongation rate at the skin-pass mill is given by the relative difference of the material speed rolled out of the skin-pass rolling stand minus the material speed rolled into the stand.

[0049] As the test progresses, measure Wa 2.5-8 Waviness value. This measurement consists in acquiring, by mechanical palpation without sliding, the profile of a sheet of length 40 mm measured in a direction transverse to the rolling direction. The long-wave component corresponding to the form is separated using a Gaussian filter with a cut-off value of 8 mm. The waviness Wa is then separated from the low-wave component including the roughness Ra by a Gaussian filter with a cut-off value of 2.5 mm. The Gaussian filter used is defined in standard ISO16610-21:2012.

[0050] Example 1: Hot stamping test

[0051] The steel plates were cut into rectangular blanks with the following dimensions: 200×250 mm 2 Each blank is then heated in a furnace at 900°C for 345 seconds to 405 seconds, depending on the material thickness. After heating, each blank is transferred to a flat tool consisting of two plates. The plates are cooled by circulating water. The temperature set point of the cooling water circuit is 17°C. The tool pressing force between the two plates is 50T.

[0052] The waviness Wa corresponding to each tempering elongation is measured for the skin-pass rolled steel plate. 2.5-8 The results are disclosed in Table 1.

[0053] Table 1

[0054]

[0055] *According to the example of the present invention, Underlined values Not according to the invention.

Claims

1. A car, wherein: At least one outer skin component or at least one semi-visible component is made of coated press-hardened steel, the coating of which before heating and press hardening contains by weight 8% to 12% silicon, not more than 3% iron and not more than 0.1% unavoidable impurities, the balance being aluminum, and wherein the coating on each side has a thickness of 10 μm to 20 μm.

2. The automobile according to claim 1, wherein: The at least one semi-visible component is selected from the group consisting of: an A-pillar, a B-pillar, a C-pillar, a roof rail, and a side rail.

3. The automobile according to claim 1 or 2, wherein: The component has a waviness Wa of less than 0.41 μm after press hardening. 2.5-8 .

4. The automobile according to claim 3, wherein: The component has a waviness Wa of less than 0.35 μm after press hardening. 2.5-8 .

5. The vehicle according to claims 1 to 4, wherein: The microstructure of the press-hardened component comprises at least 95% martensite by volume.

6. The vehicle according to claims 1 to 4, wherein: The microstructure of the press-hardened component comprises, by volume fraction, at least 50% martensite and less than 40% bainite.

7. The vehicle according to claims 1 to 4, wherein: The microstructure of the press-hardened component comprises 5% to 20% martensite, no more than 10% bainite and at least 75% equiaxed ferrite.

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